The conception of wavelet energy spectrum (WES) is posed by comparing the conservation of total energy in harmonic analysis,power spectrum analysis and wavelet transform. The calculations of a compound test series show that the WES has a fine performance in audiovisually revealing the time changes of strength of fluctuations in both time and frequency domains,especially for time changes of cos ω2t×cos ω1t type. The WES analysis of SSTA series in the tropical Pacific suggests that its interannual variability is primarily composed of biennial,quadrennial and annual oscillations,and these components simultaneously strengthen or weaken in a time interval about 10 years.